U.S. Pat. No. 4,625,993 discloses an active suspension system for an automotive vehicle. The active suspension system operates between the sprung (wheels) and unsprung (chassis) masses of the vehicle. It achieves and maintains (with no vehicle operator inputs or responses) a desired attitude of the vehicle in response to static loading on the vehicle, and minimizes the motion, acceleration and vibration of the passenger compartment caused by road and vehicle dynamics to improve vehicle handling, reduce noise, improve ride, and improve stability. These features are achieved by controlling hydraulic suspension units located at the vehicle wheels.
The system comprises a hydraulic circuit, a plurality of sensors, and an electrical control system. The hydraulic circuit communicates a pump and a hydraulic reservoir with hydraulic actuators and valves at the suspension units. The sensors sense speed, acceleration, and inputs to the vehicle, and the electrical control system controls the flow of hydraulic fluid in the actuators and valves in response to the sensed inputs
The suspension units in the active suspension system disclosed in the '993 patent each comprise a hydraulic actuator and a spring acting in parallel between a vehicle wheel and the vehicle chassis An electro-hydraulic servo valve associated with an actuator controls the flow of hydraulic fluid which effects movement of a piston in such actuator. Sensors at each suspension unit sense the load on the actuator, displacement between the wheel and the chassis, and acceleration of the wheel hub. Other sensors carried on the vehicle sense speed and acceleration of the vehicle.
The electrical control system responds to outputs from the sensors to provide displacement demand signals for each suspension unit so as to control static and dynamic displacement of the sprung and unsprung masses. The electrical control system also provides signals representing actual displacement for each suspension unit. The control system then compares the displacement demand signals with the actual displacement signals to produce error signals. Each servo valve in the hydraulic circuit is operated in accordance with the associated error signal to cause the actual displacement of the piston in the associated actuator to equal the desired displacement. The active suspension system thus controls the positions of the pistons to achieve a desired vehicle attitude by operating the servo valves.
The pump in the active suspension system disclosed in the '993 patent is driven by the vehicle engine. The hydraulic fluid power consumed at each suspension unit is equal to the product of the flow through the associated servo valve multiplied by the pressure provided by the pump. Energy is thus dissipated upon the flow of hydraulic fluid through the servo valves under pressure provided by the pump. Since the pump is driven by the vehicle engine, it is desirable to minimize energy dissipated by the active suspension system.
It is known that energy can be saved in an active suspension system by including hydraulic accumulators for effecting flows of hydraulic fluid in the actuators which would otherwise be effected by operating the servo valves. The accumulators store and dissipate energy which would otherwise be dissipated by flows of hydraulic fluid through the servo valves.